A biomechanical study of posterior glenoid bone loss and humeral head translation

Background: Results of shoulder arthroplasty have been reported to be inferior with posterior glenoid wear and accompanying subluxation of the humeral head. The purpose of this study was to examine the effect of posterior glenoid wear on posterior subluxation of the humeral head. Material and methods: A custom loading device was used to simulate physiologic loading conditions in 8 cadaver shoulders with the humerus positioned at neutral, forward flexion, and extension. Three-dimensional motion analysis recorded humeral head translation with respect to the scapula at each humerus position after removing posterior glenoid bone in 5° increments. The magnitude of humeral head translation was analyzed with 2-way ANOVA to determine the effects of arm position and glenoid condition. Results: Glenoid condition and arm position in the transverse plane significantly influenced head translation (P < .0001). With the humerus at neutral, posterior translation became significant after 20° of posterior bone removal (P < .05). However, with the humerus in forward flexion, posterior translation became significant at only 5° of posterior bone removal (P < .001). No significant differences in translation were detected for posterior defects up to 25° with the arm in extension. Conclusion: Posterior humeral head translation increased significantly with 5° of posterior glenoid bone loss, which equates to approximately 2.5° of glenoid retroversion. Awareness that humeral head translation may be seen with small amounts of retroversion should be recognized during preoperative planning for shoulder arthroplasty and when counseling the patient with regard to expected outcomes.